In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, mobile terminals, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks, which provide access to data networks, such as the Internet, and/or the public-switched telecommunications network (PSTN). The RAN covers a geographical area that is divided into cell areas, with each cell area being served by a radio base station (also referred to as a base station, a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals that are in range of the base stations.
Cellular communications system operators have begun offering mobile broadband data services based on, for example, Wideband Code Division Multiple Access (WCDMA), High Speed Packet Access (HSPA), and Long Term Evolution (LTE) wireless technologies. Moreover, fueled by introduction of new devices designed for data applications, end user performance requirements are steadily increasing. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by high-speed wireless data networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently are desired.
Techniques to improve downlink (base station-to-wireless terminal) performance may include multiple-input, multiple-output (MIMO) techniques such as 4-branch MIMO, multi-flow communication, multi-carrier deployment, etc. Since spectral efficiencies on a per-link basis may be approaching theoretical limits, some next steps for improving downlink performance will be focused on improving spectral efficiencies per unit area. Further efficiencies for wireless networks may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell. Currently, so-called heterogeneous networks are being developed by members of the 3rd-Generation Partnership Project (3GPP) as discussed, for example, in: RP-121436, Study on UMTS Heterogeneous Networks, TSG RAN Meeting #57, Chicago, USA, 4-7 Sep. 2012; R1-124512, Initial considerations on Heterogeneous Networks for UMTS, Ericsson, ST-Ericsson, 3GOO TSG RAN WG1 Meeting #70 bis, San Diego, Calif., USA, 8-12 Oct. 2012; and R1-124513, Heterogeneous Network Deployment Scenarios, Ericsson, ST-Ericsson, 3GPP TSG-RAN WG1 #70 bis, San Diego, Calif., USA, 8-12 Oct. 2012.
A traditional cellular network, which may be referred to as a “homogeneous network,” is a network of base stations (also referred to as NodeB's, enhanced NodeB's, or eNBs) in a planned layout, providing communications services for a collection of user terminals (also referred to as user equipment nodes, UEs, and/or wireless terminals). In a homogeneous network, all or most of the base stations in a given region may have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all or most of the base stations in a homogeneous network may offer unrestricted access to user terminals in the network, and each base station may be capable of serving roughly the same number of user terminals. Current cellular wireless communications systems in this category may include, for example, Global System for Mobile communication (GSM) networks, WCDMA networks, networks that support High-Speed Downlink Packet Access (HSDPA), LTE networks, Worldwide Interoperability for Microwave Access (WiMAX) networks, etc.
In a heterogeneous network, low-power base stations (also referred to as low-power nodes, LPNs, micro nodes, pico nodes, femto nodes, relay nodes, remote radio units or “RRU” nodes, small cells, etc.) may be deployed along with or as an overlay to planned and/or regularly placed macro base stations. A macro base station (MBS) may thus provide service over a relatively large macro cell area and each low-power node (LPN) may provide service for a respective relatively small LPN cell coverage area that falls all or partly within a relatively large macro cell coverage area. The radio power transmitted by an LPN (e.g., 2 Watts) may be relatively small compared to the power transmitted by a macro base station (e.g., 40 Watts for a typical macro base station). An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in the coverage provided by the macro base stations, and/or to off-load traffic from macro base stations (e.g., to increase capacity in a high traffic location, also referred to as a hot-spot). Due to the lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition.
Inter-cell interference generally presents a big performance issue for cell edge users. In a heterogeneous network (HetNet), the impact of inter-cell interference can be much higher, due to large differences between the transmit power levels of macro base stations and LPNs. This is illustrated in FIG. 1, which illustrates inter-cell interference between a macro base station 100 and an LPN 110B, at a wireless terminal 120B. The interference arises because of a transmission from the macro base station 100 to wireless terminal 120A, which may occur at the same time as transmissions from LPN 110B to wireless terminal 120B.
In the illustrated scenario, the coverage areas of the LPNs 110A and 110B fall entirely within a coverage area 130 of the macro base station. The stripe-covered regions in FIG. 1 cover a region between an outer circle and an inner circle around each of the LPNs 110A and 110B. The inner circle represents an area where the received power from each LPN 110 is higher than the received power from the macro base station 100. The outer circle represents an area where the path loss to the LPN base station 110 is smaller than that to the macro base station 100.
The stripe-covered area between the inner and outer circles is often referred to as the imbalance zone. This imbalance zone could potentially be an LPN range-expansion area because, from the uplink (terminal-to-base-station) perspective, the system would prefer that the terminal still be served by the LPN within this area. However, from the downlink (base-station-to-terminal) perspective, terminals at the outer edge of such an imbalance zone experience a very large received-power difference between the macro and LPN layers. For example, if the transmit power levels are 40 watts and 1 watt for the macro node and LPN, respectively, this power difference can be as high as 16 dB. As a result of these power differences, if a terminal in the range-expansion zone is served by a LPN cell and the macro cell is serving another terminal at the same time, using the same radio resources, then the terminal served by the LPN is subject to very severe interference from the macro base station.
In LTE, a technique known as “inter-cell interference coordination” (ICIC) is supported, via the eNodeB-to-eNodeB X2 interface. Each cell can signal to its neighboring cells, identifying high-power resource blocks in the frequency or time domains. This allows the neighboring cells to schedule cell-edge users in such a way as to avoid these high-power resource blocks. Such a mechanism can be used to reduce the impact of inter-cell interference. However, even when ICIC is supported, some inter-cell interference remains. The terminal that is interfered with is often called a “victim” terminal.
The impact of inter-cell interference, whether or not it is partly mitigated by ICIC, can depend on the victim terminal's ability to mitigate interference, which may in turn depend on the sophistication of the terminal's receiver and/or the availability of information about the interfering signal, for use in interference suppression or cancellation. For example, a victim terminal may attempt to first decode the interference signal and then cancel the interference signal from the received signal before decoding its desired signal. In many cases, even relatively strong interference signals can thus be rendered harmless, provided that they can be decoded correctly and removed at a victim terminal.
An additional technique known as “network-assisted interference cancellation” (NA-IC) is being considered in 3GPP for both LTE and HSPA networks. The concept is to have the network send assistance information to the wireless terminal (referred to as a “UE” in 3GPP documentation) to enable the terminal to perform interference cancellation. In particular, the wireless terminal needs information to allow it to decode signals that are targeted to other devices.
The scenario in FIG. 1 is one in which NA-IC could be beneficial. As shown, an LPN-served UE 120B in the range expansion area (stripe-covered area) of LNP 110B experiences strong experience from the macro base station 100. In such a scenario, if the network provides certain information about the interference signal to the victim UE 120B, then the victim UE 120B may be able to cancel the interfering macro signal and boost its achievable data rate in the LPN downlink. This assistance information may include, for example, the UE ID for the UE 120A that is the actual target of the interfering signal, the modulation format of the interfering signal, the transport block size, etc.